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Carbon Steel With Duplex SS

Carbon Steel With Duplex SS

Carbon Steel With Duplex SS


Do I need to have an Isolation member (isolation joint , isolation flange or kits) between two lines constructed from dissimilar Metals specifically Carbon Steel And Duplex Stainless Steel.
For the same situation I have three cases;
1/ Both lines are running aboveground.
2/ One line running above ground & the other underground.
3/ Both lines are running underground.

Mohd Yaseen

RE: Carbon Steel With Duplex SS

It depends on how corrosive the environment is.
The only difference that above ground or buried makes is the ability to see them.
If it is dry with no expected corrosion then don't isolate.
If you expect any chance of active corrosion then isolate.
If you don't isolate them and have active corrosion the CS near the duplex SS will suffer much accelerated attack.
Is there any possible risk of internal attack? Then you would isolate them anyway.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Carbon Steel With Duplex SS

Dear EdStainless;

thanks for detailed answer,

If I understood you well,
for point (1) no need for isolation (regardless the medium contents)
for point (2) & (3) the isolation should be dependent on the medium flow through the line

The medium flow inside the line I have two cases;
One has crude oil with a large H2S content
Another line having Crude oil with high chloride content

Mohd Yaseen

RE: Carbon Steel With Duplex SS

OK, this is not a smart-ass question.
I just would like to learn.
Why would you have a medium that requires DSS flowing through a CS pipe ?
I could understand if it came in via C/S and then was heated or cooled or had additives that then required DSS but this doesn't seem to be the case.
Not an engineer so asking in the quest to broaden my knowledge,

RE: Carbon Steel With Duplex SS


actually there is no logic, this what found from rubbish FEED Design executed by the Contractor.

Mohd Yaseen

RE: Carbon Steel With Duplex SS

Well its difficult to tell from a distance.

It might indeed be a "rubbish" FEED, but sometimes you do get mixed metals.

This might be e.g. where pipework can't be inspected or maintained or upstream of some sort of corrosion inhibitor chemicals so DSS is used initially then the CI protects the CS pipe, which probably has a fair corrosion allowance, but still hugely cheaper than DSS.

A filed I know with very high H2S does exactly this. DSS on the manifold, CI injection then CS from about 20D D/S of the injection point and where it can be regularly inspected either surface u/T or piggable sections.

The mixed metals thing is a common question in the corrosion engineering forum so that would be a good place to start, but in general the answer needs to know more data about the system.

E.g. what is the relative mass of the two items?
Is there any sort of conductive coupling between the two pipes to create a circuit.
It's probably good practice to electrically isolate the two if you can. What is the joint between them? A flange or are they welded?

Do a search for galvanic corrosion on this site and you'll find a lot of posts as it comes up frequently.
Crude Oil without much water isn't a good conductor so you may be OK.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Carbon Steel With Duplex SS

In your case, assume there will be some water present in the crude service environment and galvanic corrosion will be of concern.

Also do speak to the originator(s) of the FEED and ask pertinent questions regarding your concerns. They should be able to provide competent guidance.

RE: Carbon Steel With Duplex SS

Look at my last line.
IF there is any water in the line and any risk at all of CS corrosion then you must isolate them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Carbon Steel With Duplex SS

Thanks for all your interactions;

the joint not directly between the DSS & CS, actually the DSS line on the suction of our export pump while CS A106 Gr. B (SSC&HIC) with 3mm Corrosion Allowance on the discharge side.
As normal strainer with isolation valve in the suction line (DSS) which have flange to flange connection with each spool of the DSS, the DSS connection with CS meant for the connection with strainer, valve and the pump suction flange.

There is no conductive coupling.

the medium transported is oily water which will have a huge amount of water in.

For isolation what would you suggest?

Mohd Yaseen

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